Method of heating and cooling a wafer during semiconductor processing

ABSTRACT

A clamping ring and temperature regulated platen for clamping a wafer to the platen and regulating the temperature of the wafer. The force of the clamping ring against the wafer is produced by the weight of the clamping ring. A roof shields all but a few contact regions of the interface between the wafer and clamp from receiving depositing particles so that a coating formed on the wafer makes continuous contact with the clamping ring in only a few narrow regions that act as conductive bridges when the depositing layer is conductive.

This is a continuation of U.S. application Ser. No. 08/109,207 filedAug. 19, 1993, now abandoned, which is a continuation of U.S.application Ser. No. 08/001,994 filed Jan. 8, 1993, now abandoned whichis a division of U.S. Ser. No. 07/939,542 filed Sep. 2, 1992, now U.S.Pat. No. 5,228,501, which is a continuation of U.S. application Ser. No.07/513,318 filed Apr. 20, 1990, now abandoned, which is acontinuation-in-part of commonly assigned U.S. Ser. No. 07/760,848 filedSep. 17, 1991, now U.S. Pat. No. 5,215,196 which is a continuation ofU.S. Ser. No. 07/595,793 filed Oct. 9, 1990, now abandoned, which is acontinuation of U.S. Ser. No. 07/411,189 filed Sep. 20, 1989, nowabandoned, which is a continuation of U.S. Ser. No. 07/343,035 filedApr. 25, 1989 (now abandoned) which is a continuation of U.S. Ser. No.07/185,215 filed Apr. 25, 1988, now U.S. Pat. No. 4,842,683 which is acontinuation of U.S. Ser. No. 07/147,594 filed Jan. 22, 1988 (nowabandoned) which is a continuation of U.S. Ser. No. 06/944,843 filedDec. 19, 1986, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates in general to a wafer processing system in whichthe wafer is clamped to a platen and relates more particularly to such asystem in which the temperature of the wafer is regulated.

In many applications, it is important to control the temperature of thewafer during processing. Excessive heating of a wafer can produceexcessive diffusion of dopants within the wafer, can outgas and shrinkphotoresist that has been patterned to define the limits of waferfeatures and can segregate impurities at epitaxial interfaces. Thisproblem is increasingly important as linewidths shrink because of thereduced tolerance for undesired diffusion and because of the largenumber of processing steps that must be performed without degrading theresults of earlier steps. Excessive heating can result in numerousprocess steps including physical vapor deposition, ion implantation, ionbeam milling and reactive ion etching.

Demand for high system throughput favors the use of high power ion beamsin such steps, thereby producing an undesired rate of wafer heating.Batch process can reduce the amount of heating by keeping throughputhigh while reducing the power dissipated per wafer. However, thereduction of feature size favors the use of single wafer processingsystems. In addition, for acceptable throughput, even batch processingsystems can exhibit unacceptable levels of heating. It is thereforeimportant to include in the wafer processing system a mechanism forcooling the wafer during processing.

Conversely, numerous process steps are best performed at elevatedtemperatures. For example, elevated temperatures are useful inimplantation and diffusion steps to assist diffusion of the dopants andhealing of the lattice structure. Similarly, step coverage can beimproved by utilizing elevated wafer temperatures during depositionsteps. It is therefore important to include in the wafer processingsystem a mechanism for heating the wafer during processing. Inparticular, such wafer heating can be used to elevate the wafer to thedesired process temperature before the process step is initiated.

A number of prior systems have included heating and or cooling of thewafer during wafer processing. Early systems relied on radiative heatingand cooling to regulate the temperature of the wafer. Unfortunately,silicon wafers are relatively transparent to infrared radiation and therates of cooling and heating by radiation alone have been inadequate.

Subsequent systems clamp the wafer to a platen and regulate the wafertemperature by regulating the temperature of the platen. Unfortunately,at the microscopic level, the solid-to-solid contact between a platenand a wafer exhibits actual contact over less than 5% of the interfacebetween the wafer and platen. This is not a significant problem atnormal ambient pressures since gas molecules filling the regions betweenthe contact points provide a significant amount of thermal conduction.Unfortunately, the wafer is typically processed at extremely lowpressures so that there is only a very small contribution from the gasparticles in the spaces between the solid-to-solid contact points. Toimprove the thermal conductivity between the wafer and the platen, oneprior method utilizes a thermally conductive, pliable material betweenthe wafer and the platen.

Unfortunately, as is indicated in U.S. Pat. No. 4,261,762 entitledMethod For Conducting Heat To Or From An Article Being Treated UnderVacuum, issued to Monroe L. King on Apr. 14, 1981, this approachexhibits problems with repeatability, thermal nonuniformity andexcessively expensive maintenance. Therefore, in this system, the waferis clamped directly against the platen and a gas is provided to theinterface between the wafer and the platen to provide gas-assistedthermal transport between these two elements. This gas is providedthrough a vertical channel through the platen at a pressure of about 0.5to 2.0 Torr. The platen is cooled to remove heat from the wafer.

In U.S. Pat. No. 4,743,570 entitled Method Of Thermal Treatment Of AWafer In An Evacuated Environment, issued to Lawrence T. Lamont, Jr. onMay 10, 1988, the platen includes both heating and cooling mechanisms.The wafer is again held in direct contact with the platen by a set offour clamps that grip the wafer by its peripheral edge. However, becausethese clips are relatively flimsy, the gas pressure between the waferand platen is limited to the range 0.1-1 Torr.

In U.S. Pat. No. 4,512,391 entitled Apparatus For Thermal Treatment OfSemiconductor Wafers By Gas Conduction Incorporating Peripheral GasInlet, issued to David J. Harra on Apr. 23, 1985, the platen includes alip against which the wafer is held to produce, in the region betweenthe wafer and the platen and surrounded by the lip, a first cavitywithin which heat is transported between the platen and wafer by a gas.Gas is provided on axis into a second cavity which is connected to thefirst cavity by a plurality of inlets located just inside of the lip.This structure produces a uniform gas pressure over almost the entirevolume of the first cavity, thereby producing an equally uniform thermalconductance.

In U.S. Pat. No. 4,457,359 entitled Apparatus For Gas-assistedSolid-to-solid Thermal Transfer With A Semiconductor Wafer, issued toScott C. Holden on Jul. 3, 1984, a spring-biased clamp presses a waferagainst a domed platen with sufficient force to bend the wafer intoconforming contact with the platen. A groove in the platen just insideof the region in which the wafer is clamped, supplies gas to theinterface between the wafer and platen. This large amount ofprestressing the wafer enables a much larger gas pressure to be producedat the wafer-clamp interface without bowing the wafer away from theplaten. When the gas between the wafer and the platen is on the order ofor larger than 5 Torr, gas flow is laminar so that there is no increasein thermal conductivity above this pressure.

In U.S. Pat. No. 4,542,298 entitled Methods And Apparatus ForGas-Assisted Thermal Transfer With A Semiconductor Wafer, issued toScott C. Holden on Sep. 17, 1985, the clamp and an attached bellowsfunction as a seal between the ion bombardment chamber and a region intowhich gas leaks from between the wafer and the platen. This reduces thethe amount of gas leakage into the ion bombardment chamber.

In U.S. Pat. No. 4,671,204 entitled Low Compliance Seal For Gas-enhancedWafer Cooling In Vacuum, issued to Jon M. Ballou on Jun. 9, 1987, a newtype of sealing ring is presented that produces an adequate seal betweenthe wafer and platen without significantly stressing the wafer.

SUMMARY OF THE INVENTION

As feature size continues to decrease, it becomes more and more commonto utilize single wafer processing systems. The increasing use of singlewafer processing systems increases the importance of using largerdiameter wafers to maintain chip throughput.

For a given gas pressure between the wafer and the platen, the totalforce pushing the wafer away from the platen increases as the square ofthe diameter of the wafer. Because the gas between the wafer and theplaten is generally at a pressure within the molecular flow domain, ifthe gas pressure is reduced to avoid bowing the wafer away from theplaten, then the thermal conductivity between the wafer and platen willdecrease. Thus, it is advantageous to utilize pressures that bow thewafer away from the platen even though this requires an increasedclamping force to prevent the wafer from lifting away from the platen.

This increased clamping force increases the risk of breaking the waferby excessive local clamping pressures. At such increased force, it isimportant to be able to determine the clamping force with significantprecision so that the stress limit of the wafer is not inadvertantlyexceeded. The wide variability of spring stiffness in spring-biasedclamping systems produces an equally large uncertainty in the actualclamping force applied to the wafer. Not only are there significantdifferences in spring stiffness for apparently identical springs, inaddition, the stiffness of a given spring is significantly affected bythe temperature of the spring and the thermal history of the spring.Especially at temperatures above 250° C., the spring stiffness varieswith time. Also, a spring's stiffness varies significantly withtemperature over the approximately 500° C. temperature range that iscommon in wafer processing systems.

As is illustrated in the spring-biased clamping systems of theabove-discussed references by Holden and Ballou, one end of each springis attached to the clamp and the other end is attached to a rigid springsupport member. The amount of clamping pressure is determined by thedistance of the wafer from this spring support member. Therefore, for aselected clamping pressure, the distance of the wafer from this supportmember is uniquely determined. However, there are applications in whichit is useful to vary the position of the wafer within the processchamber. To provide this position variability for a selected clampingforce, it would be necessary to vary the position of the spring supportmember. Such variability would add unwanted complexity to the clampingsystem.

Another problem with spring-biased clamps is that the springs are asource of particulates. As the clamping force is applied to the wafer,the compression of the spring produces flaking of material from thesprings. In particular, if the springs are exposed to process gases,then surface deposits can form that will flake off when the spring isbiased.

In all of the above-discussed references, the pressure of the gasbetween the wafer and the platen is kept low enough that the pressuredoes not significantly bow the wafer away from the platen. Even in thosereferences where the wafer is bent into conformity with a domed platen,the gas pressure introduced between the wafer and the platen is selectedto be on the order of or less than the wafer-platen pressure produced byconformably clamping the wafer to the platen. Within this gas pressurerange, the wafer remains in contact with the platen across substantiallythe entire surface of the wafer.

Because the displacement of the center of the wafer from the platen is arapidly increasing function of the wafer diameter, even for a moderateelevation of the gas pressure above this limit, wafer bowing must betaken into account in large diameter wafers such as the 8" diameterwafers now commonly being used. A platen and clamp design is thereforeneeded that provides uniform clamping pressure and good thermalconduction between the wafer and platen even when the pressure bows thewafer away from the platen.

In accordance with the illustrated preferred embodiment, a platen andclamp are presented that provide repeatable, constant clamping pressureof the wafer against the platen. The shape of the platen provides goodthermal contact between the wafer and platen even when the pressure ofgas between the wafer and platen bows the wafer away from the platen.

In this wafer processing system, a movable platen is elevated to pressthe wafer into the clamp. The vertical mobility of the platen enablesthe elevation of the wafer to be varied within the process chamber,thereby adding an extra degree of freedom in the wafer fabricationprocess. The wafer is loaded onto the platen and then the distancebetween the platen and clamp is reduced until the clamp is lifted by theplaten off of a clamp support ring. The force between the clamp and thewafer is equal to the weight of the clamp and therefore is not subjectto the variability that is present in spring-biased clamping systems. Inthe preferred embodiment, the movement of the platen relative to theclamp is achieved by movement of the platen only, but in alternateembodiments it is achieved by movement of either just the clamp or boththe clamp and the platen.

Because the pressure of the clamp against the wafer is determined by theweight of the clamp, the variable clamping pressure produced byspring-biased clamping systems is eliminated. By eliminating thesprings, a significant source of particulates is also eliminated. Inaddition, in the systems in which the platen is movable, the elevationof the wafer within the process chamber during processing can becontrolled to alter processing of the wafer.

In deposition type wafer processing systems, the clamp includes a roofthat shields the portion of the wafer that is in contact with the clamp.This is done to prevent deposition onto the circular region of contactbetween the wafer and the clamp. If this were not done, then separationof the clamp from the wafer would break the deposited coating at theclamp/wafer interface, thereby producing unwanted particulates anddamage to the deposited layer. A sufficiently strong coating would alsoproduce enough bonding between the clamp and wafer that wafer breakagecould occur when the clamp is separated from the wafer. This roof alsoshields the peripheral edge of the wafer from deposition, therebyavoiding the production of particulates that would form by flaking offof this peripheral edge.

It has been observed that control over the electric potential of aconductive layer that is being deposited enables improved application ofthis conductive layer into contact openings and vias. Therefore, forclamping rings that are to be utilized for metal deposition steps, atleast one section of the clamp/wafer interface is allowed to be coatedwith the deposited metal layer. This produces one or more conductivebridges from the clamp to the deposited metal layer. An electricpotential is applied to the clamp and, because of these conductivebridges, this potential is also applied to the metal layer beingdeposited. In order to avoid arcing from the platen to the conductivelayer being deposited, the platen is shorted to the clamping ring. Thetotal area of these bridges is made large enough to to provide thecurrent needed to produce the desired electric potential of thedepositing layer. The total area of the bridges is made small enoughthat, when the clamp is lifted away from the wafer, these bridges breakwithout damaging the wafer or the deposited metal layer. In general,this is achieved for a set of six bridges, each having a width between1/8" and 3/4" wide.

Around the perimeter of the platen is a lip having an inner edgeelevated slightly above its outer edge so that a relatively gas-tightseal is produced between the wafer this inner edge. Just inside thisinner edge is a groove that encircles the inner portion of the topsurface of the platen. At least one radial groove connects thisperipheral groove to a gas inlet hole near the center of the platen.This enables gas to be provided conveniently at the center of the platenand distributed by the radial groove(s) to the peripheral groove.

The gas pressure is selected to be large enough that the wafer bows awayfrom the platen, thereby producing a shallow cavity between the waferand the platen. However, to preserve substantially constant thermalconductivity across the surface of the wafer so that uniform heating andcooling of the wafer is effected, the gap between the platen and waferis kept substantially constant. This is achieved by means of adome-shaped region inside of the peripheral lip of the platen. Thecurvature of this dome is selected to substantially match the curvatureof the wafer when the pressure of the gas between the wafer and platenis increased to a preselected maximum value.

The top of the dome-shaped region is flattened and is slightly above theheight of the lip so that a sufficiently stable support of the wafer onthe platen is produced as the platen lifts the wafer into the clampingring. This avoids wobbling of the wafer that could produce lateraland/or rotational misalignment of the wafer.

One or more heater elements are attached to the back surface of theplaten to enable heating of the wafer. Similarly, a tube carrying acoolant is thermally connected to the platen to enable cooling of thewafer. A gas inlet tube is attached to the hole in the center of theplaten to provide gas to the shallow cavity between the wafer andplaten. A temperature sensor in thermal contact with the platen providestemperature data that enables control of the wafer temperature. Thecurrent to the heater elements and the flow of cooling fluid are bothcontrolled to produce a wafer temperature appropriate for the waferprocess being implemented.

DESCRIPTION OF THE FIGURES

FIG. 1 is a side cross-sectional view of a wafer support assembly havingheating and cooling elements to regulate the temperature of a wafer.

FIG. 2A is a top view of the wafer platen.

FIG. 2B is a side cross-sectional view of the wafer platen.

FIG. 2C is an exploded view of the lip of the wafer platen.

FIG. 3 illustrates the wafer transport assembly.

FIG. 4A is a top view of the clamping ring.

FIG. 4B is a bottom view of the clamping ring.

FIG. 4C is a side cross-sectional view through a portion of the clampingring that comes into contact with the wafer lifting fingers of FIG. 3.

FIG. 4D is a side cross-sectional view through a portion of the clampingring containing a contact region.

FIG. 4E is an expanded view of the tip of a contact region, illustratingthe roof in the regions away from the contact regions.

FIG. 5 is a cross-sectional view of the wafer support assembly with theplaten in the lifted position and the wafer clamped against the platenby the clamping ring.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the figures, the first digit of a reference numeral indicates thefirst figure in which is presented the element indicated by thatreference numeral.

FIG. 1 is a side cross-sectional view of a wafer support assembly 10having heating elements 11 and 12, a cooling tube 13 and a temperaturesensor such as thermocouple 14 to regulate the temperature of a wafer15. This particular embodiment is designed to support and temperatureregulate an 8" diameter wafer. In embodiments for smaller diameterwafers, the use of a single heating element 12 is sufficient to heat thewafer. The ability to heat the wafer is useful in raising the wafertemperature to any elevated temperature at which a wafer processing stepshould be performed. The ability to cool the wafer enables thedissipation from the wafer of excess heat supplied by such high energyprocesses as physical vapor deposition, ion implantation, ion beammilling and reactive ion etching. The temperature sensor, the heatingand cooling elements and external control circuitry (not shown) regulatethe temperature of the wafer during processing.

Electrical power of up to 1000 Watts is supplied to heating elementsthrough electrical leads 16, busbar 17 and heater terminals 18. Acooling fluid such as water is supplied through cooling tube 13 at arate up to 2 liters/hour to extract heat from wafer support assembly 10.An upper heat distribution plate 19 of silver plated copper, in intimatecontact with heating elements 11 and 12 and with a stainless steelplaten 110, distributes heat uniformly from the heating elements to theplaten. A lower heat distribution plate 111 conducts heat from platen110 and heater elements to cooling tube 13. These are held in intimatecontact with heating elements by screws 112. Cooling tube 13 is ofstainless steel and is brazed to lower heat distribution plate 111 toproduce a strong thermal coupling to this distribution plate. Thethermocouple is held in intimate contact with platen 110 by means of ahollow screw 113. The electrical signal from the thermocouple istransmitted by an electrical cable 114. Elements 11-14, 16-19 and111-114 are inclosed between platen 110 and a backing plate 115 that iswelded to a tubular protrusion 116 of platen 110.

To enhance thermal coupling between wafer 15 and platen 110, a gas suchas argon is supplied through a gas inlet 117 and gas supply tube 118 toa set of three cylindrical bores 119 through platen 110. Each of thesebores connects to an associated radial groove 120 in a top surface 121of the platen. These radial grooves distribute this gas from bores 119to a circular groove 21 illustrated in FIG. 2A. This circular groove islocated just inside of an annular lip 22 at the periphery of platen 110.

FIG. 2B is a side cross-sectional view of platen 110. FIG. 2C is anexploded view of lip 22 illustrating that an inner edge 23 of lip 22 isabout 0.004 inches higher than the outer edge 24 of the lip so that ahigh pressure contact is formed between the wafer and this inner edge.This produces a solid-to-solid seal between the wafer and the platen.This seal enables a gas pressure on the order of 0.5-8 Torr to beproduced between the wafer and platen by a gas flow on the order of 5-30sccm. This pressure is sufficient to produce a sufficiently effectivethermal coupling between the wafer and the platen that the wafertemperature can be regulated by the heating and cooling capabilities ofthe wafer support assembly.

At the microscopic level, there are voids between the wafer and inneredge 23 so that gas is able to penetrate slowly through this seal intothe process chamber. However, the rate of leakage is low enough that itdoes not interfere with wafer processing.

A clamping ring 122 is brought into contact with the wafer to press thewafer against platen top surface 121 with a force equal to the weight ofthe clamping ring. By using the weight of the clamping ring as thesource of this force instead of using a spring biased clamping system asin previously discussed clamping systems, this force is accuratelydetermined and does not vary with the temperature or thermal history ofclamping springs.

In general, the clamping ring can be brought into contact with the waferby any type of relative movement between the platen and the clampingring. However, in this embodiment, this relative movement is produced byupward movement of the platen. By moving the platen, not only does thissystem enable the wafer to be brought into contact with the clampingring, it also enables variation of the elevation of the wafer within theprocessing chamber during wafer processing, thereby providing anadditional degree of freedom in the processing of the wafer.

To enable this vertical movement of platen, into the center of backingplate 115 is welded a tube 123 that is slidable vertically through acylindrical bearing 124 that is mounted in a bearing housing 125. Thishousing is attached by bolts 126 to a flange 127 that is mounted intothe bottom wall (not shown) of a wafer processing vacuum chamber.Elements 124-126 extend through a hole in the bottom of the vacuumchamber to enable electrical leads 16, electrical cable 114, coolinglines 13 and gas supply tube 118 to exit the vacuum chamber. An O-ringis held in an O-ring groove 128 in flange 127 to produce a vacuum-tightseal between the vacuum chamber wall and flange 127. A flexible metalbellows 129 is attached between backing plate 115 and flange 127 topreserve the vacuum within the vacuum chamber while enabling verticalmotion of the platen. Tube 123, bearing 124 and bellows 129 preventmotion of the platen lateral to bearing 124 or rotationally about theaxis of bearing 124.

When the platen is in a lowered position, the clamping ring is supportedon a stationary support/shield 130. A bushing 131 and associated pin 132extending through this bushing prevent lateral movement or rotationabout a vertical axis while allowing vertical movement of the clampingring. Pins 132 are attached to and part of the clamping ring. As the topof the platen is raised upward, the platen first comes into contact witha wafer 15 and lifts it upward into contact with clamping ring 122.Further upward movement of the platen lifts the clamping ring upwardproducing on the wafer a clamping force equal to the weight of theclamping ring (including the weight of pins 132).

FIG. 3 illustrates a wafer lifting assembly 30 that is used to transportthe wafer within the vacuum chamber to a point at which it can be liftedby platen 110. Wafer 15 is transported into the vacuum chamber through awafer load lock (not shown) on a wafer transport blade 31. Wafer liftingassembly 30 is positioned beneath the wafer to lift the wafer off of thewafer blade. Wafer lifting assembly 30 includes a set of four wafersupport fingers 133 that are each attached to a horseshoe shaped supportring 134 by a bolt 135. Each finger has a ledge 32 on which a portion ofthe wafer is supported as the wafer is lifted off of wafer transportblade 31. The shape of support ring 134 lets the wafer lifting assembly30 slip past the blade to lift the wafer clear of the wafer transportblade. Each finger also includes a sloping sidewall 33 that helps centerthe wafer onto the wafer lifting assembly. The wafer is then transportedon wafer lifting assembly to the position illustrated in FIG. 1.

FIGS. 4A-4E illustrate the clamping ring in greater detail. FIGS. 4A and4B are top and bottom views of clamping ring 122. FIGS. 4C and 4D arecross-sectional views of the clamping ring along the cuts indicated inFIG. 4B. The clamping ring makes contact with the wafer in six contactregions 136 that are distributed uniformly around the wafer. Thecross-section of FIG. 4D passes through a contact region 136 and thecross-section of FIG. 4C passes through a region that is not a contactregion and that is designed to receive the tips of lifting fingers 133as wafer lifting assembly 30 makes contact with the clamping ring.

As is illustrated in FIGS. 4C and 4E, in a region away from contactregions 136, a surface 41 of the clamping ring makes contact with thewafer. A roof 42 extends farther over the wafer to prevent depositiononto edge 43 of the interface between the wafer and surface 41. Ifdeposition onto the edge of this interface were allowed over a largefraction of this interface, this deposited layer could bond the clampingring to the wafer sufficiently strongly that the wafer may not bereleased after processing. Even if no wafer damage results, the shearingof such a large region would produce an undesired level of particulates.Therefore, the roof shields all portions of contact edge 43 away fromcontact regions 136 and therefore shields approximately 80% of edge 43.The spacing between the underside of the roof and the wafer issufficiently large (on the order of 0.02") that no continuous layer willbe produced from the wafer surface to the underside of the roof.

A sloping sidewall 44 and a groove 45 are included to mate with the tipof a wafer support finger 133. As the support finger makes contact withthe clamping ring, sloping sidewalls 33 and 44 come into contact andfunction to align clamping ring 122 with wafer lifting assembly 30. Thetips of the wafer support fingers fit into grooves 45 that produce astable coupling between the clamping ring and the wafer liftingassembly.

As is illustrated in FIG. 4D, within a contact region 136, undersurface41 extends clear out to the inner edge 46 of the clamping ring.Therefore, in these contact regions, edge 43 of the interface betweensurface 41 and the wafer is not protected by a roof and therefore thedepositing layer will form a continuous bridge from the wafer surface tothe clamp in these contact regions This is advantageous in the case ofconductive coatings because this enables the electric potential of thisdepositing layer to be controlled by applying a controlled voltage tothe clamping ring. It has been observed that such voltage control of adepositing conductive layer can produce improved coverage of such layerinto contact holes and vias in the wafer. To prevent arcing from thisconductive layer to the platen top surface, the clamping ring is shortedto the platen. To minimize particulate production when the clamping ringis separated from the wafer, the circumferential lengths should be assmall as possible while assuring sufficient electrical linkage acrossthe conductive bridges to controllably bias the depositing conductivelayer.

The clamping ring shields the edge of the wafer from all deposition.This is done to avoid producing on the edge of the wafer deposits thatcan flake off and produce particulates that interfere with waferprocessing. An alignment ring 47 is included in angular portions of theclamping ring away from the contact regions 136. This alignment ring hasa sloping inner surface 48 that helps center the wafer into the clampingring. This alignment ring also prevents deposition laterally beyond thisalignment ring.

This particular embodiment of clamping ring is for use with wafershaving a flat formed into the edge of the wafer to identify crystalorientation. To effectively shield the wafer edge along such a flat, itis necessary that the roof include a region 49 that shields that portionof the wafer edge. In embodiments for which the wafer will not have sucha flat, the roof will exhibit a circular profile when viewed from aboveor below.

After wafer lifting assembly 30 lifts the wafer off of the wafertransport blade, assembly 30 transports the wafer to a position asillustrated in FIG. 1. Section 34 of assembly 30 is attached to apneumatic lift (not shown) that elevates assembly 30 vertically to liftclamping ring 122 off of support/shield 130, thereby aligning theclamping ring with the wafer supported on wafer lifting assembly 30.Tube 123 is also attached to a pneumatic lift that is now activated tolift platen 110 into contact with wafer 15 and to lift this wafer awayfrom wafer support fingers 133. As the platen is elevated through thewafer position, fingers 133 pass through the indentations 25 in the lipof the platen. Further elevation of the platen brings the wafer intocontact with clamping ring 122 and lifts this ring off of supportfingers 133. When this is achieved, the clamping ring provides aclamping force equal to the weight of the clamping ring.

The top surface 121 of platen 110 is dome shaped and the weight of theclamping ring is sufficient to bend the wafer into conformity with mostof surface 121 and to press the peripheral portion of the wafer intocontact with inner edge 23 of lip 22. A gas pressure on the order of0.5-8 Torr is produced at the interface between the wafer and topsurface 121. This pressure is sufficient to bow the wafer away from thistop surface. However, the curvature of domed top surface 121 is selectedso that the gap between the wafer and surface 121 is substantiallyconstant over the entire top surface 121. The substantial constancy ofthis gap and the substantial constancy of the gas pressure within thegap produces a substantially constant rate of heat transfer across thewafer.

In the central portion of domed surface 121, the platen is intentionallyflattened to provide a local fiat surface on which the wafer rests as itis raised into the clamping ring. This fiat portion of the surfaceproduces sufficient support to avoid the rocking of the wafer that wouldresult if top surface 121 did not include this flattened region. Suchrocking could misalign the wafer on the platen. Sufficient flatness isachieved if the inner third of top surface 121 is flattened. Because ofthe large radius of curvature of the domed surface compared to thelateral dimension of this top surface, only a thin layer of materialneeds to be removed from the top surface to flatten this regions.Therefore, the gap between the wafer and platen will not besignificantly altered in this region.

We claim:
 1. A method of transferring heat to or from a wafer duringprocessing in a vacuum chamber, said method comprising the steps of:a)clamping the wafer to a platen having a domed surface that is encircledby an annular lip that is recessed relative to an inner portion of thedomed surface and protrudes relative to an outer portion of the domedsurface such that said wafer when not clamped against said annular lipcan be supported on the domed surface without making contact with theannular lip; b) producing in a volume between the wafer and the domedsurface of the platen a gas poressure high enough to bow the wafer suchthat a substantially constant gap is produced between the wafer and theplaten, whereby a gas flow path is produced having a reduced resistanceto gas flow, thereby enabling cooling or heating of the wafer byconvection.
 2. A method according to claim 1 wherein said domed surfacehas a flat central region, so that the wafer can be supported on thisflat surface without rocking during movement of the platen.
 3. A methodof aligning a wafer with respect to a support platen which comprisesa)transferring a wafer onto a support platen having a domed surface and aflat central region by means of a plurality of support fingers having asurface to support the periphery of said wafer, b) lifting said plateninto a clamping ring having a plurality of grooves that mate with saidfingers, thereby aligning said wafer with said support platen and withsaid clamping ring.
 4. A method according to claim 3 wherein saidclamping ring has an extended portion that overlies said fingers and theperiphery of said wafer, thereby shielding said wafer from depositsduring processing deposition.
 5. A method of transferring heat to andfrom a wafer during processing in a vacuum chamber, comprisinga) loadinga wafer onto a vertically movable platen having a domed surface with aflat central region into said vacuum chamber, b) changing the relativeposition of the platen having the wafer supported thereon and a clampingring for said wafer so that the clamping ring engages the periphery ofthe wafer, and c) producing in a volume between the wafer and the domedand flattened surface of the platen a constant gas pressure high enoughto bow the wafer and produce a substantially constant gap between thewafer and the platen, whereby a gas flow path is established sufficientto enable cooling or heating of the wafer by convection.
 6. A methodaccording to claim 5 wherein the platen has an annular lip around theperiphery of the domed surface and the periphery of the wafer is engagedby the inner edge of said lip by the clamping ring.
 7. A methodaccording to claim 5 wherein the gas pressure produced between the waferand the surface of the platen is from about 0.5-8 Torr.
 8. A methodaccording to claim 5 wherein said change in position comprises liftingthe platen until the wafer supports the full weight of the clampingring.